Conventionally, magneto-optical storage media have been commercially manufactured as rewritable optical storage media. A drawback of the magneto-optical storage medium is that its reproduction properties deteriorate with a decrease in the size of the recording bit (magnetic recording domain) and in the interval between adjacent recording bits, relative to the size of a light beam that is emitted from a semiconductor laser device and then converged on the magneto-optical storage medium.
This is because the light beam converged on the targeted recording bit encompasses adjacent recording bits within its coverage and fails to separately reproduce the individual recording bits.
To overcome the drawback, various magnetic super high resolution reproduction technologies have been developed using a magnetic multi-layer film. These magnetic super high resolution reproduction technologies reduces interference between plus and minus signals during reproduction by forming a magnetic masking area and thus forming a magnetic aperture that is smaller than the beam spot, and enables reproduction of signals whose cycles do not exceed diffraction limits of light.
Nevertheless, the magnetic super high resolution reproduction technologies have a problem that the strength of reproduced signals decreases with a decrease in the recording cycle for the magnetic recording domain, because the aperture also needs to be reduced in size.
To solve the problem, a method is suggested to enable magnetic domain expansion reproduction without applying a.c. external magnetic fields (Magnetic Domain Expansion Readout with DC laser and DC magnetic field [Magnetic Amplifying Magneto-Optical System, or MAMMOS]), an article from resumes for lectures in 44th Conference organized in spring 1997 by the Society of Applied Physics Researchers, 30a-NF-3, page 1068).
Now, referring to FIG. 28 through FIG. 30, a magneto-optical storage medium based on the method will be explained. FIGS. 28 and 29 are plan and cross-sectional views schematically illustrating magnetization of the magneto-optical storage medium during reproduction. FIG. 30 is a cross-sectional view showing the medium arrangement of a magneto-optical disk that is an application of the magneto-optical storage medium.
As shown in FIG. 29, the magneto-optical storage medium is arranged from stacked layers including a reproduction layer 201, a supplementary reproduction layer 202, and a storage layer 204. The reproduction layer 201 and the supplementary reproduction layer 202 exhibit an in-plane magnetization state at room temperature, and changes to a perpendicular magnetization state as temperature is elevated by projection of a converged light beam 205 (light beam spot 205' in FIG. 28). The storage layer 204 is made of a perpendicular magnetization film, where magnetic information is stored as magnetization directions in magnetic domains 206 and 207.
The reproduction layer 201 is specified to change to a perpendicular magnetization state at a temperature lower than the temperature at which the supplementary reproduction layer 202 changes to a perpendicular magnetization state. Consequently, on heating using the light beam 205, the magnetic domain 209 where the reproduction layer 201 has changed to a perpendicular magnetization state grows larger than the magnetic domain 208 where the supplementary reproduction layer 202 changes to a perpendicular magnetization state.
The magnetization direction in the magnetic domain 208, where the supplementary reproduction layer 202 changes to a perpendicular magnetization state due to the heating with the light beam 205, is determined by coupling with the storage layer 204 through exchange forces. Hence, the magnetic information in the magnetic domain 206 in the storage layer 204 is duplicated to the supplementary reproduction layer 202 so that the direction of the auxiliary grating moment of the supplementary reproduction layer 202 conforms to that of the storage layer 204.
Next, the magnetic information in the magnetic domain 208, where the supplementary reproduction layer 202 has changed to a perpendicular magnetization state, is duplicated to the reproduction layer 201 so that the direction of the transition metal (TM) moment of the reproduction layer 201 conforms to that of the supplementary reproduction layer 202. Here, since the magnetic domain 209, where the reproduction layer 201 changes to a perpendicular magnetization state, grows larger than the magnetic domain 208, where the supplementary reproduction layer 202 changes to a perpendicular magnetization state, the magnetization state of the supplementary reproduction layer 202, i.e., the magnetization state of the storage layer 204, is amplified and duplicated to the reproduction layer 201.
As described above, in the magneto-optical storage medium in accordance to the aforementioned method, the magnetic information in the storage layer 204 is amplified and duplicated to the reproduction layer 201; therefore magnetic recording domains with a reduced recording cycle still allows reproduction of strong signals.
It should be noted that as shown in FIG. 30 the magneto-optical storage medium, having the arrangement shown in FIG. 29, constitutes a magneto-optical disk when stacked together with a substrate 210, a transparent dielectric protective layer 211, and a protective layer 212.
However, since the storage layer 204, the supplementary reproduction layer 202, and the reproduction layer 201 are coupled together through exchange forces, the transition from an in-plane magnetization state to a perpendicular magnetization state of the supplementary reproduction layer 202 and the reproduction layer 201 proceeds gradually with rising temperature; therefore the magneto-optical storage medium used for magnetic domain expansion reproduction in accordance with the method still has a problem that reproduction resolution is difficult to improve.
Further, the supplementary reproduction layer 202 and the reproduction layer 201 need to be thick so that the transition from an in-plane magnetization state to a perpendicular magnetization state of the supplementary reproduction layer 202 and the reproduction layer 201 takes place with rising temperature in a stable manner; however, greater thicknesses of the layers degrade playback sensitivity, which is yet another problem.